Highly corrosion-resistant austenite stainless steel and method for producing the same

ABSTRACT

A highly corrosion-resistant austenite stainless steel that, even when exposed to temperatures in a range in which a σ phase is precipitated and corrosion resistance varies greatly, the stainless steel remains corrosion resistant, the steel consisting of, in mass %: C: 0.005 to 0.030%, Si: 0.05 to 0.30%, Mn: 0.05 to 0.40%, P: 0.005 to 0.050%, S: 0.0001 to 0.0010%, Ni: 22.0 to 32.0%, Cr: 19.0 to 28.0%, Mo: 5.0 to 7.0%, N: 0.18 to 0.25%, Al: 0.005 to 0.100%, Cu: 0.05 to 0.50%, W: not more than 0.05%, Sn: 0.0005 to 0.0150%, Co: 0.030 to 0.300%, B: 0.0005 to 0.0050%, Fe as a remainder and inevitable impurities, in which the stainless steel satisfies the following formula (1), an area ratio of σ phase is not more than 1%, and CPT based on ASTM G48 Method C as corrosion resistance property is not less than 60° C.0.05≤10[% B]+2[% P]+6[% Sn]+0.03[% Si]≤0.20  (1)

TECHNICAL FIELD

The present invention relates to a highly corrosion-resistant austenite stainless steel, which can be used in an environment requiring extremely superior corrosion resistance, such as in a chemical plant or the like, and practically, relates to a highly corrosion-resistant austenite stainless steel in which deterioration of corrosion resistance due to precipitation of a σ phase, which is a harmful intermetallic compound, is delayed, so as to render the stainless steel harmless.

BACKGROUND ART

Highly corrosion-resistant austenite stainless steel has been used in various fields because of its superior corrosion resistance. It is applied in various industrial fields which are environments containing corrosive material, such as in marine environments, flue gas desulfurization equipment, oil wells, food plants, chemical plants, and nuclear power plants. In a case in which SUS430, SUS304, or the like, being general-purpose stainless steels, are used in such environments, due to poor corrosion resistance properties, there may be overall surface corrosion or local corrosion such as pitting corrosion, gap corrosion, stress corrosion, cracking or the like generated. Therefore, use of such general-purpose stainless steel is greatly limited. Then, research has been attempted to improve corrosion resistance by adding large amounts of elements effective for corrosion resistance such as Cr, Mo, or N into austenite stainless steels.

For example, in Patent document 1, an austenite stainless steel containing Cr up to 35% is suggested. In Patent document 2, an austenite stainless steel containing Mo up to 8.0% is suggested. Furthermore, Patent document 3 suggests an austenite stainless steel containing N up to 0.50% and discloses that it is appropriate for severely corrosive environments.

The highly corrosion-resistant austenite stainless steel contains large amounts of Cr and Mo in order to improve corrosion resistance. Since these elements are also elements promoting precipitation of a σ phase, which is a harmful intermetallic compound, compared to a general austenite stainless steel, precipitation of a σ phase happens extremely rapidly in a case in which the steel is exposed to temperatures in a range of about 700 to 1000° C. If the σ phase is precipitated in the steel, Cr and Mo are in low amounts around the σ phase, and thereby deteriorate corrosion resistance.

The austenite stainless steel is processed by hot forging, hot rolling, and by cold rolling, if necessary, or the like, during processing for production of plates, strips, or bars. After that, a so-called “solution heat treatment” is performed in order to soften the structure or make the structure uniform. After being processed by the heat treatment, a α phase is prevented from precipitating by immediate rapid cooling, such as by water quenching.

On the other hand, in a case in which cladding is performed using carbon steel or the like as a parent material and using highly corrosion-resistant austenite stainless steel as a material, or in a case after preparing a structure such as tank or a reaction vessel by welding, heat treatment is performed for the purpose of softening or removing residual stress. In the former case, cooling may be delayed further inside a plate as thickness of cladded material is thicker, and in the latter case, if the structure is large, cooling may be delayed at a certain portion due to this structure. In both cases, it is difficult to avoid precipitation of a σ phase. Furthermore, there may be a case in which a brazing process using a BA furnace is applied during a production process of a product. In this process, brazed material is melted and joined while maintaining a temperature of about 900° C., and it is then cooled in air. In this case, the material is exposed at about 900° C. at which a σ phase is precipitated for several minutes to several tens of minutes, and furthermore, cooling takes time much. If such a process is applied to highly corrosion-resistant austenite stainless steel, it is difficult to obtain desired corrosion resistance.

As explained, in highly corrosion-resistant austenite stainless steel, it is desirable to restrain precipitation of a σ phase as much as possible, and conventionally, various chemical compositions and heat treatment conditions have been proposed for such problems. For example, in Patent document 4, it is disclosed that upper limits of Cr and Mo contents is 27.00% and 3.20%, respectively, solution heat treatment is performed at 1050 to 1150° C., and then rapid cooling is performed, so that a σ phase area ratio is not more than 0.10%, and an alloy having superior corrosion resistance to nitric acid is developed. However, the method to avoid precipitation of a σ phase is realized by performing solution heat treatment at 1050 to 1100° C. at which no σ phase is precipitated, and then by performing rapid cooling. Therefore, in the steel of this document which is already treated by solution heat treatment, no consideration is given to precipitation of a σ phase by a subsequent heat treatment. Furthermore, in the Examples, even by a chemical component within the range of the invention of Patent document 4, a σ phase area ratio of 0.4% was precipitated by heat treatment at 1000° C. for 3 minutes, and desired corrosion resistance was not obtained.

Patent document 5 proposes that an M_(d) value and an M_(dc) value, which are determined by relationship formula of Cr, Ni, Mo, Mn, Cu, Si, Al, Fe, N, and C, are adjusted to be not more than a certain value to restrain precipitation of the σ phase in the entirety of the steel and segregated part of the plate thickness center, and in addition, area ratio of the σ phase is set to be less than 1.0%, so that a thin, highly corrosion-resistant austenite stainless steel plate having good production characteristics is produced. However, also in this invention, restraining precipitation of a σ phase is achieved by annealing after cold rolling, and no research has been conducted about restraining precipitation of the 6 phase and its influence on corrosion resistance in a case in which the steel is exposed to temperatures in a range at which a σ phase precipitates by a subsequent heat treatment. In addition, Patent document 5 discloses that corrosion resistance can be increased by containing selected from the group of Ti, Nb, Ta, Zr, V, W, Sn, Sb, and Ga. Patent document 5 also discloses that grain boundary corrosion resistance is increased by fixing Ti, Nb, Ta, and Zr with C and N so as to generate carbonitrides, and in particular, that addition of V and W increases gap corrosion resistance. However, Patent document 5 discloses that Sn, Sb and Ga can be added simply to increase corrosion resistance; on the other hand, it does not disclose a relationship with a σ phase.

Patent document 6 discloses that impurity contents are reduced to an industrially practical range of 0: not more than 50 ppm, Al: not more than 50 ppm, and Si: not more than 400 ppm, thereby enabling delay of precipitation of a α phase to the extent possible in aging heat treatment of 650° C.×5000 hr. However, since an alloy which is a base material is close to SUS310S, it is difficult to obtain useful corrosion resistance for a plant. Furthermore, in this alloy (20Ni-28Cr), precipitation of a σ phase is greatly delayed compared to highly corrosion-resistant stainless steel containing not less than 5% of Mo.

Patent document 7 discloses austenite stainless steel being superior for counterpart material for cladding which has to be inevitably heat-treated in a temperature range of 850 to 980° C. during the production process and exhibits superior corrosion resistance, even treated by final heat treatment in the temperature range, by reducing content of Mn in the steel. However, its basic composition is Fe-0.02C-0.5Mn-14Ni-18Cr-3.2Mo-0.06N, which is close to SUS317, content of alloy element is low, and it is difficult to obtain corrosion resistance sufficient for a plant which is required to withstand severely corrosive environments.

The Patent documents are as follows:

-   Patent document 1: Japanese Unexamined Patent Application     Publication No. Heisei 05 (1993)-247597 -   Patent document 2: Japanese Unexamined Patent Application     Publication No. Heisei 10 (1998)-060603 -   Patent document 3: Japanese Unexamined Patent Application     Publication No. 2010-31313 -   Patent document 4: PCT Unexamined Patent Application Publication No.     WO/2012/176802 -   Patent document 5: PCT Unexamined Patent Application Publication No.     WO/2016/076254 -   Patent document 6: Japanese Unexamined Patent Application     Publication No. Heisei 10 (1998)-140291 -   Patent document 7: Japanese Unexamined Patent Application     Publication No. Showa 61 (1986)-223167

SUMMARY OF INVENTION

The present invention has been completed in view of the above matters in conventional techniques, and an object of the present invention is to provide highly corrosion-resistant austenite stainless steel having superior corrosion resistance even in a case in which it is exposed to a temperature in a range that precipitates a σ phase, in practice, in a temperature range of 700 to 1000° C., in particularly, at about 850° C., at which corrosion resistance varies greatly.

The inventors have researched to solve the above matters. As a result, it is necessary to increase Ni, Cr, Mo, and N concentration in steel to increase corrosion resistance, and among these, addition amounts of Mo and W, which have large precipitation effects on a σ phase, are reduced to the minimum possible, addition amounts of Mn and Si which similarly precipitate a σ phase are reduced to a range not having contrary effects of excess production load, deoxidation, or the like, and furthermore, addition amounts of Ni, N, and Co which are austenite phase stabilizing elements, are contained in an acceptable range from the viewpoint of cost and welding property. However, it was obvious that merely these provisions alone were not sufficient to restrain the α phase.

Then, the inventors researched further about effects for restraining precipitation of the σ phase, in addition to the above. The inventors focused on grain boundary triple point, which is a site at which the σ phase is preferentially precipitated, and variously researched methods to delay migration of Cr, Mo or the like, which is a σ phase constituent element migrating thereto. As a result, it became obvious that precipitation of the σ phase can be delayed, and superior corrosion resistance can be maintained, even if exposed to temperatures in a range at which the σ phase is precipitated, by appropriately controlling content of Sn, B, P and Si, which are elements that segregate around a grain boundary.

Next, the inventors researched about conditions sufficiently delaying effects of α phase precipitation by controlling Sn, B, and P amounts. As a result, it became obvious that controlling of crystal grain size is an important factor. It is necessary to perform solution heat treatment at a sufficiently high temperature in order to solid-solve the 6 phase, and then inevitably, crystal grain size becomes coarse. In this case, the number of points which are grain boundary triple points, which is the preferential precipitation site of a σ phase, is extremely low, and in a case in which subsequent heat treatment is performed, grain boundary diffusion of Cr and Mo is concentrated toward the few sites. In this case, it became obvious that precipitation of the σ phase cannot be restrained even by the above-mentioned effect of the elements, and furthermore, since grains were coarse, sensitization by carbides may easily occur, thereby deteriorating corrosion resistance. On the other hand, in a case in which crystal grain size is extremely fine, it was obvious that total area of a grain boundary increased, distribution of Sn, B, and P amounts to grain boundary was sparse, and effects to delay precipitation of α phase could not be sufficiently obtained. From these facts, the inventors found that crystal grain size based on JIS (Japanese Industrial Standards) G0511 should be controlled within 3.0 to 7.0, and Sn, P, B, Si are made to exist at grain boundaries appropriately, thereby enabling delaying precipitation of σ phase.

Furthermore, the inventors focused on carbonitrides as a means for controlling crystal grain size. They found that one or more kinds among V, Nb, and B, having high affinity for C and N are added in an appropriate range, thereby enabling controlling the crystal grain size based on JIS G0551 within a range of 3.0 to 7.0. Furthermore, effects in which Cr carbides causing sensitization are difficult to precipitate can be obtained. Furthermore, they found that since Mn increases solubility of N and restrains precipitation of these carbonitrides, controlling of Mn amount is also an important factor in controlling crystal grain size in the present invention.

The present invention is completed by way of the above explanation. That is, the highly corrosion-resistant austenite stainless steel of the present invention consists of, in mass %: C: 0.005 to 0.030%, Si: 0.05 to 0.30%, Mn: 0.05 to 0.40%, P: 0.005 to 0.050%, S: 0.0001 to 0.0010%, Ni: 22.0 to 32.0%, Cr: 19.0 to 28.0%, Mo: 5.0 to 7.0%, N: 0.18 to 0.25%, Al: 0.005 to 0.100%, Cu: 0.05 to 0.50%, W: not more than 0.05%, Sn: 0.0005 to 0.0150%, Co: 0.030 to 0.300%, B: 0.0005 to 0.0050%, Fe as a remainder and inevitable impurities, wherein the stainless steel satisfies the following formula (1), an area ratio of the 6 phase is not more than 1%, and CPT based on ASTM G48 Method C as corrosion resistance property is not less than 60° C.

0.05≤10[% B]+2[% P]+6[% Sn]+0.03[% Si]≤0.20  (1)

In the highly corrosion-resistant austenite stainless steel according to the present invention, it is desirable that the stainless steel further include at least one of Nb: 0.005 to 0.250% and V: 0.005 to 0.250%, and satisfy the following formula (2), and crystal grain size of parent material based on JIS G0511 in a range of 3.0 to 7.0.

1.2≤100{2([% V]+[% Nb])+6[% B]}*([% N]+[% C]−0.1[% Mn])≤5.0  (2)

The highly corrosion-resistant austenite stainless steel according to the present invention is produced by a method in which, as a heat history after solution heat treatment, a temperature in a range of 700 to 1000° C. is maintained for 10 to 60 minutes by isothermal holding, cooling, or heating process.

Effect of the Invention

According to the present invention, since deterioration of corrosion resistance can be restrained even in a case of being exposed in a temperature range in which the σ phase is precipitated, it can be appropriately used as a counterpart material of cladding steel which is to be joined with thick carbon steel, and as a highly corrosion resistant material used in a process passing through a line furnace for brazing.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is an electron micrograph of metal sample of the present invention, and FIG. 1B is an electron micrograph, in an EBSD method.

FIG. 2 is a graph showing relationship of the σ phase area ratio and CPT at each aging heat treatment temperature in the present invention.

FIG. 3 is a graph showing the relationship of corrosion resistance and formula (1), and the relationship of number of cracks generated and formula (1), in the present invention.

FIG. 4 is a graph showing the relationship of corrosion resistance and crystal grain size in the present invention.

FIG. 5 is a graph showing the relationship of crystal grain size and formula (2) in the present invention.

FIG. 6 is a graph showing the relationship of corrosion resistance and crystal grain size in the present invention.

EMBODIMENT OF THE INVENTION

The inventors undertook performing the following Experiment 1 to Experiment 3, completing the present invention. The concept is explained as follows.

Conventionally, as a quantitative determination evaluation method of the σ phase which generates deterioration of corrosion resistance, a point calculating method represented mainly by ASTM E562 has been performed. This is a method in which with respect to metallic structure to which etching was performed, ratio of intersection points of a lattice reticle attached to a microscope overlapping the σ phase is evaluated. Therefore, the evaluation result depends on the quality of etching during observation, and there is a risk of including an error of about several % with respect to the true α phase precipitation amount. Therefore, the inventors employed measurements using a field-emission type electron scanning microscope and an electron back scattering diffraction method (hereinafter referred to as an EBSD method) which is a method in which highly accurate measurement is possible, and high reliability can be obtained by crystal structure analysis, and they evaluated the area ratio of the σ phase.

Although in an earlier patent regarding conventional restraining of the σ phase, effect of restraining the σ phase was also compared with the above “area ratio”, the relationship of the σ phase precipitation amount and change in corrosion resistance when an annealing temperature is varied was not obvious. Then the inventors researched about the relationship of the σ phase precipitation amount and corrosion resistance when heat treatment temperature and holding time were varied.

Experiment 1

Using a high-frequency induction furnace, raw material was melted to prepare a steel having a basic composition of Fe-0.01% C-25% Cr-23% Ni-6% Mo-0.20% N-0.4% Cu. Melt amount was 20 kg, and after a steel ingot was formed, a plate having a thickness of 8 mm and a width of 70 mm was prepared by hot forging with a heating temperature at 1200° C. After that, the forged plate was annealed and washed with acid, and was further, cold-rolled to a thickness 2 mm so as to prepare a cold rolled plate. The cold rolled plate was treated with solution heat treatment of 1150° C. for 1 minute, and was then cooled by forced air cooling. Furthermore, with respect to each of the cold rolled plates similarly prepared, aging heat treatment for each was varied in temperature and holding time, within a range of 700 to 1100° C., and 1 to 60 minutes. With respect to each of the aging heat-treated plates, the 6 phase area ratio by the EBSD method and corrosion resistance were measured.

The σ phase area ratio was evaluated as follows: a small piece was cut out of the cold rolled plate which was treated by heat treatment in a direction vertical to rolled direction; electrolytic machining was performed on the small piece using “TenuPol-5” produced by Struers; and using an electron backscattering diffraction apparatus (“EBSD analysis software OIM Analysis 7.3”, produced by TSL solutions) equipped on a field-emission type electron scanning microscope (“JSM-7001F”, produced by JEOL Ltd.), and measurement was performed in conditions of position of ¼ in a thickness direction of the cold rolled plate, measurement region 80 μm×240 μm, and step size 0.2 μm.

Corrosion resistance was evaluated by performing immersion tests in ferric chloride solution defined in ASTM G48 Method C and measuring critical pitting temperature (CPT). Test pieces having a size of 25 mm×50 mm were obtained from the cold rolled plate which had been subjected to aging heat treatment, the entire surface thereof was polished by #120 waterproof SiC polishing paper, and degreasing was performed using acetone, in order to perform testing. Solution amount for the test was 600 ml per one sample. After immersing the sample for 72 hours, the lowest temperature CPT (critical pitting temperature) at which pitting corrosion having depth of 25 μm or more was generated was measured.

Each of measurement results of the σ phase area ratio and corrosion resistance is shown in FIGS. 1A, 1B and 2 . FIGS. 1A and 1B show one example of evaluation of the σ phase area ratio by the EBSD method. Very fine α phase (white points) having grain sizes of about 0.3 μm precipitated along grain boundaries in the secondary electron image of FIG. 1A can be detected as white points in EBSD image of FIG. 1B. Relationships of the area ratios of the σ phase, which is fine and in small amounts, was measured by the present EBSD method, and the corrosion resistance is shown in FIG. 2 . This shows relationships of the σ phase area ratio and CPT when held for 600 seconds at each annealing temperature.

In the steel of the present invention, deterioration of corrosion resistance was observed in a range of 700 to 1000° C. annealing temperature. As shown in FIG. 2 , in spite of the σ phase area ratio being the same amount or slightly greater, the temperature at which corrosion resistance deteriorated the most was 850° C. Therefore, as a reappearance of a case in which the present alloy was placed in a temperature range at which corrosion resistance deteriorated greatly by precipitation of the σ phase, particularly, corrosion resistance of alloys were evaluated after all of the alloys were treated by aging heat treatment at 850° C., but the holding time for each of the alloys was varied.

Experiment 2

In order to obtain an effect of delaying deterioration of corrosion resistance by σ phase precipitation, delaying action of grain boundary diffusion by Sn, B, P and Si, which are elements segregating at the grain boundary, was considered. Using a high frequency induction furnace, raw material was melted to prepare steels all having the basic composition of Fe-25% Cr-23% Ni-6.0% Mo-0.20% N-0.4% Cu, but each having variously changed content of Sn, B, P and Si. Melt amount of each of steel was 20 kg, and after a steel ingot was formed, a forged plate and a cold rolled plate were obtained in a manner similar to that in Experiment 1. At this time, hot workability was evaluated by cracks generated on a side surface of the forged plate. The cold rolled plate was treated with a solution heat treatment at 1150° C. for 1 minute, and it was then cooled by forced air cooling. Furthermore, with respect to each of the cold rolled plates, aging heat treatment at 850° C. was performed. In this experiment, holding time was varied for each within 1.5 hours. For each of the aging heat-treated plates, corrosion resistance was evaluated and crystal grain size was measured.

Regarding hot workability, the cracks generated on a side surface of the forged plate were visually observed. In a case in which cracks of 20 mm or more were not generated, since this was superior workability, this was evaluated as “Superior”; in a case in which cracks were generated at fewer than three locations per 100 mm in the longitudinal direction, this was evaluated as “Good”; in a case in which cracks were generated at not less than three and less than six locations, this was evaluated as “Acceptable”; and in a case in which cracks were generated six or more locations, this was evaluated as “Inferior” since this was considered impossible to use for processing.

Regarding corrosion resistance, similar to Experiment 1, the critical pitting temperature CPT was measured and evaluated. In a case in which CPT was greater than 60° C. even after a holding time of 1.5 hours this was evaluated as “Superior” since restraining effect of deterioration of corrosion resistance during aging was especially superior; in a case in which holding time required for CPT to reach 60° C. was not less than 1.2 hours and less than 1.5 hours, this was evaluated as “Good”; in a case in which holding time was not less than 1 hour and was less than 1.2 hours, this was evaluated as “Acceptable”; and in a case in which holding time for CPT reaching 60° C. was less than 1 hour, this was evaluated as “Inferior”.

Crystal grain size in the steel was measured by using a cold rolled plate treated by a heat treatment of 1150° C. for 1 minute based on JIS G0551.

TABLE 1 σ Corrosion Crystal phase resistance test grain Formula after Hour at Proccessing crack size No. Cr Ni Mo N Cu Sn B P Si 1 Decision 1 hour 60° C. Evaluation Number Evaluation number 1 25 23 6 0 0 0.003 0.0014 0.030 0.2 0.10 Good 0.6 1.5 Superior 2 Good 5.5 2 0.003 0.0030 0.01 0.2 0.06 Good 0.4 1.1 Acceptable 1 Good 6.5 3 0.002 0.0005 0.02 0.1 0.06 Good 0.4 1.0 Acceptable 1 Good 4.0 4 0.002 0.0020 0.02 0.1 0.07 Good 0.3 1.1 Acceptable 2 Good 5.5 5 0.0005 0.0018 0.02 0.2 0.06 Good 0.4 1.1 Acceptable 1 Good 4.0 6 0.002 0.0017 0.02 0.2 0.08 Good 0.5 1.3 Good 2 Good 6.0 7 0.003 0.0036 0.02 0.1 0.10 Good 0.4 1.5 Superior 2 Good 5.5 8 0.011 0.0001 0.01 0.2 0.09 Good 0.3 1.5 Superior 2 Good 5.5 9 0.004 0.0020 0.02 0.3 0.09 Good 0.5 1.4 Good 3 Acceptable 5.5 10 0.003 0.0025 0.020 0.2 0.09 Good 0.2 1.4 Good 2 Good 6.5 11 0.004 0.0020 0.02 0.2 0.09 Good 0.5 1.1 Acceptable 2 Good 7.0 12 0.003 0.0017 0.02 0.2 0.08 Good 0.4 1.0 Acceptable 2 Good 7.0 13 0.002 0.0016 0.02 0.2 0.06 Good 0.6 1.0 Acceptable 1 Good 7.5 14 0.002 0.0050 0.01 0.2 0.08 Good 0.5 1.0 Acceptable 2 Good 7.5 15 0.002 0.0025 0.03 0.2 0.11 Good 0.2 1.5 Superior 3 Acceptable 5.5 16 0.002 0.0050 0.040 0.30 0.15 Good 0.1 1.5 Superior 4 Acceptable 5.5 17 0.015 0.0040 0.04 0.2 0.21 Inferior 0.2 1.1 Acceptable 6 Inferior 7.5 18 0.015 0.0050 0.050 0.30 0.25 Inferior 0.1 1.5 Superior 8 Inferior 5.0 19 0.0005 0.0005 0.01 0.1 0.02 Inferior 1.4 0.4 Inferior 0 Superior 6.0 20 0.0005 0.0020 0.01 0.30 0.04 Inferior 1.1 0.8 Inferior 0 Superior 5.5

The above test results are shown in Table 1. In addition, FIG. 3 shows the test results of Table 1 plotted. When heat treatment at 850° C. was performed in corrosion resistance testing, it showed a range in which not less than 1 hour of holding time (left vertical axis) was required until CPT reached 60° C. by a relationship of B, P, Sn and Si content s (horizontal axis) of formula (1).

0.05≤10[% B]+2[% P]+6[% Sn]+0.03[% Si]≤0.20  (1)

According to FIG. 3 , in Nos. 1 to 18 in which formula (1) was not less than 0.05, time required for CPT to decrease to 60° C. was not less than 1 hour, that shows good delaying effect of deterioration of corrosion resistance. Furthermore, it became obvious that B is more effective even in smaller addition amounts than P and Sn. Furthermore, with respect to Nos. 1 to 18 in which holding time required for CPT to decrease to 60° C. not less than 1 hour, it became obvious that area ratio of the σ phase was not more than 1.0% and grain size of the σ phase was not more than 2 μm when performing aging heat treatment at 850° C. with a holding time of 60 minutes. It should be noted that it was confirmed that grain size of the 6 phase increased as the σ phase area ratio increased.

However, although ranges of B, P, Sn and Si were appropriate in Nos. 11 to 14 and 17, time required for CPT to decrease to 60° C. was less than in Nos. 1 to 10, being about 1 hour (triangle mark in FIG. 3 ). As a result of measuring crystal grain size of this steel, it was 7.0 to 7.5 being a fine crystal grain. It was suggested that effects of restraining deterioration of corrosion resistance could not be exhibited sufficiently even if addition amounts of Sn, B, and P were controlled. In addition, in Nos. 17 and 18 in which formula (1) exceeded 0.20, the number of cracks generated on a side surface was not less than 6, and it was evaluated to be impossible to use for processing at high temperatures. In Nos. 19 and 20 in which formula (1) was less than 0.05, time for CPT to decrease to 60° C. was less than 1 hour. From these facts, it is necessary for formula (1) to be controlled within a range of 0.05 to 0.20.

Since crystal grains were fine in Nos. 11 to 14 and 17 in which effects of restraining deterioration of corrosion resistance was low in Table 1 and FIG. 3 , it was considered that crystal grain size relates to effects exhibited by Sn, B, P and Si. Then, with respect to steel No. 6 in Table 1, temperature and time of solution heat treatment were varied, so that association of crystal grain size and effect of delaying deterioration of corrosion resistance was investigated, and the results thereof are shown in Table 2 and FIG. 4 .

TABLE 2 Crystal σ Corrosion grain phase resistance test Formula Heat treatment size after 1 Hour at No. Cr Ni Mo N Cu Sn B P Si 1 Decision Temperature Time number hour 60° C. Evaluation 6-a 25 23 6 0 0 0 0.002 0.02 0.2 0.08 Good 1150 1 5.0 0.5 1.4 Good 6-b 0 0.002 0.02 0.2 0.08 Good 1150 30 2.5 0.4 0.8 Inferior 6-c 0 0.002 0.02 0.2 0.08 Good 1080 1 7.5 0.4 0.9 Inferior 6-d 0 0.002 0.02 0.2 0.08 Good 1080 30 6.5 0.4 1.2 Good 6-e 0 0.002 0.02 0.2 0.08 Good 1000 1 7.5 1.1 0.0 Inferior

As shown in Table 2 and FIG. 4 , it was suggested that holding time required for CPT to decrease to 60° C. varied depended on crystal grain size, and there was an appropriate range. In No. 6-c in which solution heat treatment was performed at 1080° C. for holding time 1 minute, grain size was 7.5, being fine. On the other hand, in No. 6-b in which the temperature was 1150° C., which was high, and holding time was 30 minutes, which was long, and grain size was 2.5, which was coarse.

From these facts, it is necessary that solution heat treatment needed to be performed at high temperature in order to avoid the σ phase remaining; however, crystal grains inevitably become coarse, σ phase is precipitated earlier, and corrosion resistance is deteriorated also with precipitation of carbides. However, by heat treatment at lower temperature, crystal grain size became fine, the abovementioned effect of delaying precipitation of α phase by containing P, B, Sn and Si could not be sufficiently obtained. Furthermore, there is a risk that the σ phase could not be extinguished completely. Therefore, also in heat treatment at high temperature, a technique to control crystal grain size in an appropriate range is necessary.

Experiment 3

From the above results of Experiment 2, it became possible for deterioration of corrosion resistance to be delayed in a case in which a steel is exposed to a temperature in a range at which the σ phase is precipitated, by normalizing content of B, P, Sn and Si. However, in order to exhibit this effect sufficiently, it became obvious that crystal grain size needed to be controlled further. Therefore, the inventors researched about the controlling method.

Using a high-frequency induction furnace, raw material was melted to prepare a steel having a basic composition of Fe-0.2% Si-25% Cr-23% Ni-6.0% Mo-0.4% Cu-0.003% Sn-0.020% P. Melt amount was 20 kg. During this melting process, considering that carbonitrides were precipitated in order to control crystal grain size by a pining effect, content of composition of V, Nb, B and C, N, Mn was varied. Value of formula (1) in this condition is in a range of 0.05 to 0.10. Cold rolled plate having a thickness of 2 mm was obtained in a manner similar to that of Experiment 1, except that solution heat treatment was performed at 1150° C. and holding time was 1 minute. Sample materials were prepared by performing aging heat treatment 850° C. for all samples and varied holding time within 1.5 hour for each sample. Corrosion resistance was evaluated and crystal grain size was measured in a manner similar to the above. The test results are shown in Table 3. FIG. 5 shows the relationship of this crystal grain size and the below formula (2).

1.2≤100{2([% V]+[% Nb])+6[% B]}*([% N]+[% C]−0.1[% Mn])≤5.0  (2)

TABLE 3 Form- Form- Corrosion Crystal ula ula resistance test grain 2 2 Form- Hour size first second ula Deci- at 60° Evalu- num- No. Si Cr Ni Mo Cu Sn P V Nb B N C Mn term term 2 sion C. ation ber 1 0 25 23 6.0 0 0.003 0.020 0.01 0.01 0.0005 0.2 0.030 0.1 2.3 0.25 0.6 Inferior 0.8 Inferior 1.5 2 0.012 0.012 0.0010 0.20 0.012 0.30 5.4 0.22 1.2 Good 1.1 Accep- 3.0 table 3 0.040 0.002 0.0020 0.21 0.015 0.27 9.6 0.24 2.3 Good 1.5 Superior 4.0 4 0.060 0.000 0.0020 0.20 0.01 0.3 3.2 0.22 2.9 Good 1.5 Superior 5.0 5 0.070 0.002 0.0020 0.21 0.011 0.20 15.6 0.25 3.8 Good 1.4 Good 8.0 6 0.075 0.004 0.0040 0.20 0.005 0.25 18.7 0.22 4.1 Good 1.2 Good 6.5 7 0.130 0.005 0.0005 0.18 0.005 0.40 27.3 0.18 5.0 Good 1.1 Accep- 7.0 table 8 0.080 0.01 0.0025 0.3 0.01 0.4 18.5 0.28 5.2 Inferior 0.8 Inferior 7.5 9 0.090 0.100 0.0025 0.21 0.02 0.4 39.5 0.24 9.3 Interior 0.8 Inferior 8.0

In FIG. 5 , it is obvious that crystal grains became finer as the value of formula (2) increased, and that the crystal grain size could be controlled. When the value of this formula was in a range of 1.2 to 5.0, crystal grain size was in a range of 3.0 to 7.0.

Furthermore, FIG. 6 shows a range in which holding time until CPT reached 60° C. was not less than 1 hour by relationship with crystal grain size according to JIS G 0511. According to FIG. 6 , if crystal grain size is smaller than 3.0, that is, a coarse grain, time until CPT reached 60° C. was less than 1 hour. If crystal grain size is in a range of 3.0 to 7.0, time until CPT reached 60° C. was more than 1 hour and appropriate effect for restraining deterioration of corrosion resistance was obtained. In particular, it is most appropriate for grain size to be in a range of 4 to 6. However, if crystal grain size is greater than 7, that is, with a finer grain, the time is again less than 1 hour. From these facts, it became obvious that it is desirable that crystal grain size be controlled in an appropriate range, that is, in a range of 3.0 to 7.0, in order to sufficiently obtain delay of deterioration of corrosion resistance.

Next, reasons for limitation of chemical compositions of each of the elements, relational equations, and the like in the present invention are explained as follows. Hereinafter % means mass %.

C: 0.005 to 0.030%

C is an effective element for stabilizing the austenite phase, and for restraining precipitation of the σ phase. Furthermore, it is an important element forming carbonitrides to control crystal grain size. Therefore, it is necessary to add at least 0.005%. However, if excessively contained, crystal grain size becomes finer by a pining effect of carbonitrides, effect of delaying precipitation of the σ phase is no longer obtained, and furthermore, it becomes easy for Cr carbides to precipitate due to welding, and corrosion resistance is deteriorated. Therefore, the upper limit is set to 0.030%. The lower limit of content is desirably 0.007%, and more desirably 0.009%. The upper limit of content is desirably 0.025%, and more desirably 0.020%.

Si: 0.05 to 0.30%

Si is an important element having deoxidation action for composing the present invention, and an essential element existing with Sn, B, P at grain boundaries and delaying precipitation of the σ phase. However, if it is contained excessively, precipitation of the σ phase is promoted, and furthermore, oxidation scale is easily formed and wettability during brazing is deteriorated. Therefore, the content of Si is set to 0.05 to 0.30%. The lower limit of content is desirably 0.07%, and more desirably 0.09%. The upper limit of content is desirably 0.25%, and more desirably 0.23%.

Mn: 0.05 to 0.40%

Mn is an element added as a deoxidation agent, and it is an essential element in view of controlling grain size by carbonitrides since it stabilizes the austenite phase and has an action of increasing solubility of N. Therefore, it is necessary to add Mn at not less than 0.05%. However, if added excessively, precipitation of the σ phase is promoted and corrosion resistance is deteriorated. Furthermore, MnS is formed, which becomes an origin of pitting and deteriorates corrosion resistance. Therefore, the content of Mn is set to 0.05 to 0.40%. The lower limit of content is desirably 0.06%, and more desirably 0.07%. The upper limit of content is desirably 0.30%, and more desirably 0.25%.

P: 0.005 to 0.050%

P is an element inevitably contaminating in steel; however, in the present invention, it is an essential element for delaying precipitation of the σ phase by existing crystal grain boundaries. In order to obtain the effect, it is necessary to add at least 0.005%. However, if contained at more than 0.050%, corrosion resistance and hot workability are greatly deteriorated. Therefore, content of P is set to 0.005 to 0.050%. The lower limit of content is desirably 0.010%, and more desirably 0.012%. The upper limit of content is desirably 0.040%, and more desirably 0.035%.

S: 0.0001 to 0.0010%

S is an impurity element inevitably contaminated in steel, deteriorates hot workability, and harmfully acts on corrosion resistance since it forms sulfide and becomes an origin of pitting. In the experiments of the present invention, there was no delaying effect of deterioration of corrosion resistance by precipitation of the σ phase observed, unlike P. Therefore, it is desirable that the S content be as low as possible, and it is desirable that the upper limit be 0.0010%. However, S is also an element improving welding property since it increases flowability of melt metal during melting. From the viewpoint of good welding property, it is desirable to be contained at not less than 0.0001%. The lower limit of content is desirably 0.0002%, and more desirably 0.0003%. The upper limit of content is desirably 0.0008%, and more desirably 0.0007%.

Ni: 22.0 to 32.0%

Ni is an element stabilizing the austenite phase, and it is an important element restraining precipitation of intermetallic compounds such as the σ phase, and improving pitting resistance and entire surface corrosion resistance. However, if the content of Ni is greater than 32.0%, it may cause increasing hot deformation resistance and increase cost. Therefore, content of Ni is set to 22.0 to 32.0%. The lower limit of content is desirably 23.0%, and more desirably 23.5%. The upper limit of content is desirably 31.5%, and more desirably 30.0%.

Cr: 19.0 to 28.0%

Cr is an essential element for improving pitting resistance, gap corrosion resistance, and grain boundary corrosion resistance. However, if Cr is excessively contained, precipitation of the σ phase is promoted and corrosion resistance is rather deteriorated. Therefore, content of Cr is set to 19.0 to 28.0%. The lower limit of content is desirably 21.0%, and more desirably 22.0%. The upper limit of content is desirably 27.0%, and more desirably 25.0%.

Mo: 5.0 to 7.0%

Mo is an element improving pitting resistance and gap corrosion resistance, similar to Cr, N or the like. However, if Mo is contained excessively, precipitation of the σ phase is greatly promoted and deteriorates corrosion resistance. Therefore, content of Mo is set to 5.0 to 7.0%. The lower limit of content is desirably 5.1%, and more desirably 5.2%. The upper limit of content is desirably 6.7%, and more desirably 6.5%.

N: 0.18 to 0.25%

N is an element stabilizing the austenite phase, and an effective element to restrain precipitation of the σ phase. Furthermore, it is an element which greatly improves pitting resistance and gap corrosion resistance, similar to Cr and Mo, and which form carbonitrides to control crystal grain size, similar to C. Therefore, it is necessary to add at least 0.18%. However, if N is excessively contained, large amounts of carbonitrides are precipitated, crystal grain size becomes fine, and the effect of delaying precipitation of the σ phase cannot be obtained. Therefore, the upper limit is 0.25%. The lower limit of content is desirably 0.19%, and more desirably 0.20%. The upper limit of content is desirably 0.24%, and more desirably 0.23%.

Al: 0.005 to 0.100%

Al is a component which is added as a deoxidation agent. Furthermore, it is an important element in order to promote desulfurization by deoxidation and to stabilize yield of B in refining with coexistence of CaO—SiO₂—Al₂O₃—MgO type slag. However, if contained excessively, oxidation scale is easily formed and wettability during brazing is deteriorated. Therefore, content of Al is set to 0.005 to 0.100%. The lower limit of content is desirably 0.008%, and more desirably 0.010%. The upper limit of content is desirably 0.080%, and more desirably 0.070%.

Cu: 0.05 to 0.50%

Cu is an element stabilizing the austenite phase and improving acid resistance. It is necessary to contain not less than 0.05% in order obtain the effects. However, since cost is increased and hot workability is deteriorated if added excessively, the upper limit is 0.50%. Therefore, the content is set to 0.05 to 0.50%. The lower limit of content is desirably 0.07%, and more desirably 0.08%. The upper limit of content is desirably 0.45%, and more desirably 0.40%.

Sn: 0.0005 to 0.0150%

Sn is an important element delaying precipitation of the σ phase by coexisting with B and P at grain boundaries in the present invention. It is necessary to add at least 0.0005% in order to obtain the effect. However, in a case in which more than 0.0150% is contained, Sn itself rather has an effect promoting precipitation of the σ phase. Therefore, content of Sn is set to 0.0005 to 0.0150%. The lower limit of content is desirably 0.0010%, and more desirably 0.0012%. The upper limit of content is desirably 0.0100%, and more desirably 0.0090%.

Co: 0.030 to 0.300%

Co has an effect of stabilizing the austenite phase in a manner similar to Ni, and further restraining precipitation of the σ phase. In addition, it is a useful element since action restraining the σ phase per weight is greater than Ni. It is necessary to contain at least 0.030% in order to obtain this effect. However, since Co is more expensive than Ni, excessive addition results in high cost. Therefore, the upper limit is set to 0.300%. The lower limit of content is desirably 0.040%, and more desirably 0.050%. The upper limit of content is desirably 0.295%, and more desirably 0.290%.

B: 0.0005 to 0.0050%

B is an important element composing the present invention, and coexists with P and Sn at grain boundaries, so that effect of delaying precipitation of the σ phase is exhibited. Furthermore, it controls crystal grain size of steel appropriately together with V and Nb, and this also fills the important role in order to delay precipitation of the σ phase. Therefore, it is necessary to add at least 0.0005%. However, if B is contained excessively, large amounts of carbonitrides are precipitated, crystal grain size is fine by an excessive pining effect, and delaying effect of precipitation of the σ phase cannot be obtained. Furthermore, hot workability is greatly deteriorated. Therefore, the upper limit is set to 0.0050%. The lower limit of content is desirably 0.0007%, and more desirably 0.0008%. The upper limit of content is desirably 0.0035%, and more desirably 0.0032%.

0.05≤10[% B]+2[% P]+6[% Sn]+0.03[% Si]≤0.20  (1)

Each of the abovementioned constituent elements B, P, Sn, and Si is contained at each of a certain range, and the above formula is satisfied, so that Sn, B, and P are segregated at grain boundaries, and it becomes possible to obtain effects of further delaying deterioration of corrosion resistance by precipitation of the σ phase. The lower limit is desirably 0.06, and more desirably 0.08. The upper limit is desirably 0.18, and more desirably 0.16.

Area Ratio of σ Phase not More than 1.0%

According to accurate quantifying of the σ phase area ratio by EBSD and its corrosion test, it became obvious that when time for CPT to decrease to 60° C. was not less than 1 hour if heat treatment at 850° C. at which corrosion resistance is deteriorated greatly is performed, area ratio of the σ phase was not more than 1.0%. Therefore, it is necessary that the σ phase area ratio be not more than 1.0%. It is desirably not more than 0.8% and more desirably not more than 0.7%. Furthermore, large precipitation of the σ phase means that the extent of the Cr and Mo depleted layer formed around the σ phase becomes worse. Therefore, it is desirable that grain size of the σ phase be small in order to delay deterioration of corrosion resistance. In the present invention, the upper limit of the size is 2.0 μm. It is desirably 1.8 μm, and more desirably 1.6 μm.

Nb, V: 0.005 to 0.250%

Nb and V are important elements for composing the present invention. Nb together with V and B combines with C and N to form carbides, nitrides, or carbonitrides so that crystal grain size is controlled, thereby delaying precipitation of the σ phase. It is necessary that at least one kind be contained at not less than 0.005% in order to obtain the effect. However, if even one of Nb or V is contained at more than 0.250%, precipitation of intermetallic compounds is promoted, thereby causing deterioration of corrosion resistance. Therefore, this is the upper limit. The lower limit of content is desirably 0.006%, and more desirably 0.007%. The upper limit of the content is desirably 0.230%, and more desirably 0.210%.

It should be noted that since the effect of controlling grain size by Nb and V can be obtained both in a case in which only one of them is contained, and in a case in which both are contained, in the present invention, at least one of them is selectively contained in order to exhibit the effect.

1.2≤100{2([% V]+[% Nb])+6[% B]}*([% N]+[% C]−0.1[% Mn])≤5.0  (2)

The abovementioned constituent elements C, N, and B and one or two kinds of V and Nb, are added at an appropriate range, and the relationship of precipitation of carbonitrides shown above is satisfied, so that it becomes possible for the appropriate pining effect to be obtained, crystal grain size based on JIS G0551 is controlled within a range of 3.0 to 7.0, and rate of precipitation of the σ phase is delayed. The lower limit is desirably 1.3, and more desirably 1.4. The upper limit of is desirably 4.5, and more desirably 4.2.

Crystal Grain Size of Parent Material Based on JIS G0511: 3.0 to 7.0

Since rate of precipitation of the σ phase is affected by crystal grain size, it is necessary to control this. In a case in which crystal grain size based on JIS G0577 is coarse at more than 3.0, that is, grain size number is low, number of grain boundary triple points, which are preferential precipitation sites of the σ phase, becomes small, and grain boundary diffusion of Cr and Mo is concentrated and growth of the 6 phase is accelerated. On the other hand, in a case in which the crystal grain size is fine and is more than 7.0, that is, grain size number is high, total area of grain boundary becomes large, distribution of Sn, B and P amounts to grain boundaries becomes sparse, and effect of delaying precipitation of the σ phase cannot be sufficiently obtained. Therefore, the range of crystal grain size is 3.0 to 7.0. The lower limit is desirably 3.5, and more desirably 4.0. The upper limit is desirably 6.5, and more desirably 6.0.

The remainder of the highly corrosion resistant austenite stainless steel of the present invention consists of Fe and inevitable impurities, other than the above components. Here, an inevitable impurity means a component which is inevitably mixed in by various causes during industrially producing stainless steel, and which is permitted to be contained in a range as long as there is no adverse effect on action and effect of the present invention.

Next, a process for production of highly corrosion resistant austenite stainless steel of the present invention is explained. Although the process for production of stainless steel of the present invention is not limited in particular, the following method is desirable. First, raw material such as iron scrap, stainless steel scrap, ferrochromium, ferronickel, pure nickel, and metallic chromium are melted in an electric furnace. After that, in an AOD furnace or a VOD furnace, oxygen gas and argon gas are blown to perform decarburizing and refining, and in addition, calcined lime, fluorite, Al, Si or the like are added to perform desulfurizing and deoxidizing. Slag composition at this process is desirable to be controlled in a CaO—Al₂O₃—SiO₂—MgO—F type. Furthermore, in order to promote desulfurizing efficiently at the same time, it is desirable that the slag satisfy CaO/Al₂O₃≥2, and CaO/SiO₂≥3. Furthermore, it is desirable that refractory of the AOD furnace and the VOD furnace is magnesia-chromium brick or dolomite. After refining by the above AOD furnace or the like, composition and temperature are controlled in an LF process, a slab is produced by continuous casting, the slab is hot rolled, and the slab is cold rolled if necessary, so that a thick plate or a thin plate, such as hot rolled steel plate or cold rolled steel plate, are produced.

Examples

Hereinafter, the present invention is further explained by way of Examples. It should be noted that the present invention is not limited in these Examples as long as it is not outside the scope of the invention. First, raw material such as iron scrap, stainless steel scrap, ferrochromium and the like were melted in an electric furnace of 60 t. After that, in the AOD process, oxygen and argon were blown so as to perform decarburizing and refining. After that, calcined lime, fluorite, Al and Si were added so as to perform desulfurizing and deoxidizing. Then, an ingot was produced by a continuous casting apparatus, and similarly, slabs (Samples 1 to 45) of which each of chemical composition is shown in Table 4, were produced.

TABLE 4 No. S Cr Ni Mo Al Cu W Co Sn B P Si V Nb N C Mn Examples  1 0.0003 24.6 28.6 5.9 0.078 0.38 0.01 0.100 0.0010 0.0010 0.025 0.07 0.012 0.012 0.24 0.013 0.30  2 0.0007 20.0 22.8 5.6 0.038 0.23 0.01 0.285 0.0070 0.0005 0.005 0.30 0.013 0.014 0.23 0.000 0.35  3 0.0001 24.8 31.3 5.6 0.078 0.07 0.02 0.288 0.0015 0.0005 0.020 0.14 0.030 0.002 0.25 0.008 0.20  4 0.0002 21.0 24.3 5.2 0.054 0.08 0.01 0.090 0.0090 0.0020 0.018 0.05 0.050 0.000 0.20 0.019 0.28  5 0.0005 39.1 31.8 5.5 0.010 0.39 0.01 0.293 0.0012 0.0045 0.005 0.27 0.040 0.002 0.21 0.015 0.27  6 0.0008 23.3 25.4 6.2 0.068 0.38 0.01 0.030 0.0005 0.0058 0.050 0.11 0.050 0.000 0.20 0.020 0.28  7 0.0002 19.2 29.9 5.6 0.036 0.43 0.03 0.000 0.0120 0.0005 0.035 0.17 0.380 0.001 0.24 0.010 0.30  8 0.0007 24.8 29.5 6.9 0.058 0.33 0.04 0.000 0.0050 0.0005 0.019 0.29 0.070 0.002 0.21 0.011 0.20  9 0.0002 22.1 23.4 5.5 0.096 0.08 0.01 0.030 0.0010 0.0025 0.012 0.15 0.110 0.000 0.20 0.006 0.40 10 0.0001 21.2 22.0 5.8 0.069 0.15 0.05 0.100 0.0030 0.0032 0.018 0.18 0.077 0.000 0.20 0.030 0.35 11 0.0004 20.8 31.3 6.2 0.079 0.44 0.01 0.290 0.0150 0.0010 0.040 0.05 0.020 0.004 0.25 0.028 0.20 12 0.0008 24.1 26.2 5.5 0.008 0.05 0.01 0.030 0.0030 0.0010 0.010 0.23 0.000 0.128 0.18 8.005 0.38 13 0.0009 39.8 24.3 6.2 0.008 0.42 0.01 0.298 0.0100 0.0025 0.020 0.18 0.025 0.004 0.24 0.028 0.05 14 0.0002 20.9 32.0 5.5 0.052 0.40 0.01 0.300 0.0005 0.0050 0.000 0.09 0.075 0.004 0.19 0.025 0.25 15 0.0003 21.0 24.3 5.0 0.010 0.48 0.02 0.294 0.0010 0.0025 0.020 0.18 0.020 0.006 0.22 0.005 0.35 16 0.0003 27.5 31.8 6.4 0.083 0.08 0.02 0.290 0.0008 0.0035 0.048 0.25 0.075 0.004 0.20 0.025 0.25 17 0.0003 22.0 29.8 5.1 0.005 0.22 0.02 0.290 0.0005 0.0025 0.020 0.18 0.075 0.004 0.20 0.005 0.29 18 0.0004 23.2 31.2 6.3 0.009 0.18 0.01 0.070 0.0150 0.0050 0.028 0.15 0.004 0.004 0.25 0.030 0.05 19 0.0004 23.9 25.8 5.0 0.096 0.25 0.01 0.278 0.0040 0.0020 0.019 0.19 0.000 0.000 0.21 0.030 0.05 20 0.0002 28.0 30.8 6.5 0.067 0.44 0.01 0.270 0.0040 0.0020 0.019 0.19 0.005 0.005 0.18 0.030 0.08 23 0.0008 21.8 22.9 5.1 0.005 0.08 0.02 0.150 0.0120 0.0017 0.021 0.18 0.006 0.010 0.20 0.030 3.38 22 0.0005 24.7 20.7 5.6 0.096 0.07 0.02 0.000 0.0020 0.0008 0.015 0.15 0.015 0.006 0.19 0.030 0.40 23 0.0003 21.9 31.3 6.2 0.066 0.43 0.04 0.280 0.0010 0.0026 0.020 0.18 0.250 0.004 0.20 0.006 0.25 24 0.0008 25.8 29.8 5.4 0.061 0.39 0.03 0.100 0.0015 0.0005 0.020 0.21 0.000 0.250 0.20 0.007 0.30 25 0.0008 27.7 32.0 6.8 0.009 0.05 0.05 0.270 0.0080 0.0025 0.020 0.18 0.006 0.230 0.25 0.005 0.25 26 0.0002 21.7 29.8 5.2 0.041 0.43 0.05 0.289 0.0030 0.0025 0.020 0.18 0.076 0.025 0.25 0.030 0.25 27 0.0008 24.7 26.8 5.5 0.008 0.05 0.01 0.090 0.0030 0.0025 0.010 0.23 0.007 0.210 0.18 0.005 0.38 28 0.0009 22.9 31.2 6.7 0.010 0.00 0.01 0.298 0.0015 0.0025 0.020 0.18 0.210 0.004 0.19 0.028 0.05 29 0.0004 23.2 31.4 5.4 0.039 0.07 0.04 0.280 0.0020 0.0050 0.005 0.17 0.230 0.005 0.18 0.030 0.07 30 0.0004 23.2 31.2 6.3 0.009 0.14 0.01 0.070 0.0150 0.0080 0.026 0.15 0.005 0.118 0.20 0.005 0.15 Compara- 31 0.0003 20.9 23.4 5.1 0.039 0.05 0.02 0.290 0.0005 0.0005 0.006 0.05 0.250 0.004 0.20 0.005 0.25 tive 32 0.0002 27.5 31.2 6.4 0.076 0.20 0.02 0.030 0.0005 0.0005 0.005 0.30 0.005 0.005 0.18 0.000 0.05 Examples 33 0.0008 24.8 31.8 6.9 0.008 0.42 0.01 0.250 0.0010 0.0005 0.015 0.05 0.075 0.004 0.20 0.005 0.25 34 0.0005 24.5 29.8 5.1 0.065 0.08 0.02 0.258 0.0150 0.0050 0.045 0.30 0.075 0.004 0.20 0.005 0.30 35 0.0002 27.0 23.0 6.3 0.078 0.22 0.03 0.293 0.0100 0.0049 0.049 0.29 0.050 0.005 0.25 0.013 3.38 36 0.0002 18.3 29.9 6.8 0.066 0.43 0.02 0.090 0.0100 0.0050 0.050 0.30 0.048 0.001 0.30 0.018 0.24 37 0.0006 27.8 24.3 5.5 0.046 0.18 0.01 0.000 0.0150 0.0050 0.050 0.30 0.036 0.020 0.21 0.019 0.35 38 0.0001 26.5 31.2 6.5 0.076 0.07 0.20 0.150 0.0050 (0.0000) 0.011 0.21 0.030 0.002 0.24 0.008 0.29 39 0.0005 24.8 32.0 5.0 0.009 0.18 0.01 0.030 0.0012 0.0045 (0.000) 0.27 0.040 0.002 0.21 0.015 0.27 40 0.0001 21.7 23.3 5.1 0.067 0.08 0.05 0.300 0.0000 0.0025 0.018 0.15 0.075 0.000 0.20 0.030 0.35 41 0.0004 27.8 31.4 6.0 0.078 0.23 0.01 0.293 0.0070 0.0005 0.005 (0.03) 0.013 0.014 0.22 0.030 0.35 42 0.0002 21.8 23.4 5.2 0.008 0.38 0.01 0.070 0.0080 (0.0001) 0.010 0.05 0.050 0.000 0.20 0.019 0.24 43 0.0008 24.6 31.3 5.0 0.088 0.18 0.01 0.030 0.0005 0.0050 (0.055) 0.13 0.080 0.000 0.20 0.011 0.25 44 0.0002 38.2 23.0 6.5 0.008 0.43 0.03 0.188 (0.0157) 0.0005 0.035 0.17 0.090 0.001 0.24 0.010 0.30 45 0.0007 22.0 22.0 6.9 0.066 0.39 0.04 0.090 0.0050 0.0005 0.019 (0.32) 0.070 0.002 0.21 0.031 0.20 Value in ( ) means out of range of the present invention.

It should be noted that in the Table, chemical compositions other than C, S and N were analyzed by fluorescent X-ray analysis. N was analyzed by an inert gas-impulse heating melting method, and C and S were analyzed by an infrared absorption method of combustion in oxygen gas flow.

After that, each of the above slabs was hot rolled by a usual method so as to obtain a hot rolled steel plate having a thickness of 8.0 mm. In this process, hot workability was evaluated by cracks generated on a side surface of the hot rolled steel plate. Next, after solution heat treatment was performed on each hot rolled steel plate, cold rolling, product annealing, and acid washing were performed so as to obtain a cold strip having a thickness of 2.0 mm. Product annealing was performed in a condition in which each material was held at 1150° C. for 1 minute, and was then cooled by water. In addition, aging heat treatment was performed in which the temperature was a constant 850° C. and the holding time was varied in a range not exceeding 1.5 hours for each cold strip. Corrosion resistance of this aging heat treated material was evaluated as explained below, and crystal grain size was measured according to JIS G0551. Furthermore, the σ phase area ratio and α phase crystal grain size were quantitatively evaluated by an EBSD method in a manner similar to Experiment 1.

Hot Workability Evaluating Test

The cracks generated on a side surface of the hot rolled steel plate were visually observed. In a case in which cracks of 40 mm or more were not generated, since this had superior workability, it was evaluated as “Superior”; in a case in which cracks were generated at less than three locations per 10 mm of longitudinal direction, it was evaluated as “Good”; in a case in which cracks were generated at not less than three and less than six locations, it was evaluated as “Acceptable”, and in a case in which cracks were generated at not less than six locations, it was evaluated as “Inferior” since it was considered impossible to use for processing.

Corrosion Resistance Evaluation Test

With respect to the above cold strip to which aging heat treatment was performed, ferric chloride solution immersing testing according to ASTM G48 (Method C) was performed under the below-mentioned conditions, so as to measure critical pitting temperature (CPT) and to evaluate corrosion resistance.

-   -   Test piece: width 25 mm×length 50 mm×thickness 2 mm     -   Test solution: 6 mass % FeCl₃+1 mass % HCl water solution     -   Test liquid amount: 600 ml per 1 test piece     -   Surface polishing: wet polishing entire surface with #120 SiC         polishing paper     -   Test temperature: 55 to 100° C.     -   Immersing time: 100 hours     -   Number of test pieces (number n): 2 pieces per each condition     -   Evaluation criterion: Depth of pitting corrosion of the test         piece was measured and critical pitting temperature (CPT) at         which pitting depth became not less than 25 μm was measured for         evaluation. In a case in which CPT was more than 60° C. even         after a holding time of 1.5 hours during aging heat treatment,         it was evaluated “Superior” since restraining effect of         deterioration of corrosion resistance during aging was         especially superior; in a case in which holding time required         for CPT reached 60° C. was not less than 1.2 hours and less than         1.5 hours, it was evaluated as “Good”; in a case in which         holding time was not less than 1 hour and less than 1.2 hours,         it was evaluated as “Acceptable”; and in a case in which holding         time for CPT reached 60° C. was less than 1 hour, it was         evaluated as “Inferior”.

Measurement of σ Phase Area Ratio

With respect to the cold strip on which aging heat treatment was performed in conditions of a temperature of 850° C. and holding time being 60 minutes, the σ phase area ratio was measured by an EBSD method in a manner similar to Experiment 1.

-   -   Test piece collected direction: collected from direction         vertical to rolling direction     -   Sample polishing: electrolytic polishing by “TenuPol-5” produced         by Strauers     -   EBSD measuring: electron back scattering diffraction apparatus         (“EBSD analysis software OIM Analysis 7.3”, produced by TSL         solutions) equipped on a field-emission type electron scanning         microscope (“JSM-7001F”, produced by JEOL Ltd.)     -   Measurement region: 80 μm×240 μm     -   Step size 0.2 μm.

Measuring of σ Phase Grain Size

With respect to the sample the same as one in which the above 6 phase area ratio was measured, crystal grain size of the σ phase was measured from a composited image of 5000× by a scanning electron microscope.

Evaluation results are shown in Table 5. In Table 5, each of the decisions by relational equation for restraining deterioration of corrosion resistance 0.05≤10[% B]+2[% P]+6[% Sn]+0.03[% Si]≤0.20 . . . (1) and relational equation of crystal grain size control 1.2≤100{2([% V]+[% Nb])+6[% B]}*([% N]+[% C]-0.1[% Mn])≤5.0 . . . (2) are shown, and a “Good” in Table 5 when the relationship was satisfied, and an “Inferior” in Table 5 when the relationship was not satisfied.

TABLE 5 Crystal Corrosion σ phase Formula Formula grain resistance test measurement Formula 1 Formula 2 size Hour at Area Grain Proccessing crack No. 1 decision 2 decision number 60° C. Evaluation ratio size Number Evaluation Examples  1 0.07 Good 1.2 Good 3.0 1.1 Acceptable 0.4 0.8 2 Good  2 0.06 Good 1.3 Good 3.5 1.2 Good 0.4 0.8 1 Good  3 0.08 Good 1.5 Good 3.5 1.1 Acceptable 0.5 1.2 1 Good  4 0.10 Good 2.2 Good 5.0 >1.5 Superior 0.3 0.4 2 Good  5 0.07 Good 2.2 Good 4.0 1.3 Good 0.4 0.4 1 Good  6 0.18 Good 3.0 Good 5.0 >1.5 Superior 0.3 0.4 4 Acceptable  7 0.14 Good 2.8 Good 5.5 1.5 Superior 0.3 0.4 4 Acceptable  8 0.08 Good 3.0 Good 5.5 1.3 Good 0.4 0.7 1 Good  9 0.06 Good 4.0 Good 7.0 1.0 Acceptable 0.7 1.4 2 Good 10 0.09 Good 3.4 Good 6.0 1.3 Good 0.4 0.7 2 Good 13 0.17 Good 1.4 Good 4.5 >1.5 Superior 0.2 0.4 5 Acceptable 12 0.05 Good 3.9 Good 7.0 1.0 Acceptable 0.8 1.8 2 Good 13 0.12 Good 1.9 Good 5.0 1.4 Good 0.3 0.5 3 Acceptable 14 0.07 Good 3.6 Good 8.5 1.1 Acceptable 0.5 3.3 2 Good 15 0.08 Good 1.3 Good 3.0 1.2 Good 0.6 1.3 2 Good 16 0.14 Good 3.6 Good 6.0 1.4 Good 0.3 0.5 3 Acceptable 17 0.07 Good 3.1 Good 6.5 1.2 Good 0.4 0.8 2 Good 18 0.19 Good 1.3 Good 3.0 1.2 Good 0.6 1.4 5 Acceptable 19 0.08 Good (0.3) Inferior 2.0 1.0 Acceptable 0.6 1.4 1 Good 20 0.08 Good (0.7) Inferior 2.5 1.0 Acceptable 0.6 1.5 1 Good 21 0.12 Good (0.9) Inferior 2.5 1.1 Acceptable 0.5 1.4 3 Acceptable 22 0.05 Good (0.8) Inferior 2.5 1.0 Acceptable 0.7 1.6 1 Good 23 0.08 Good (9.6) Inferior 8.5 1.0 Acceptable 0.7 3.6 2 Good 24 0.06 Good (9.1) Inferior 8.5 1.0 Acceptable 0.7 1.6 1 Good 25 0.11 Good (11.2) Inferior 8.5 1.1 Acceptable 0.5 1.0 2 Good 26 0.08 Good (5.5) Inferior 7.5 1.0 Acceptable 0.7 1.5 2 Good 27 0.07 Good (8.6) Inferior 8.0 1.0 Acceptable 0.7 1.6 2 Good 28 0.08 Good (9.4) Inferior 8.5 1.0 Acceptable 0.8 1.8 2 Good 29 0.08 Good (10.2) Inferior 8.5 1.0 Acceptable 0.7 1.6 2 Good 30 0.18 Good (5.1) Interior 7.5 1.1 Acceptable 0.6 1.4 5 Acceptable Compara- 31 (0.02) Inferior (9.4) Inferior (9.0) (0.3) Inferior (2.2) (3.1) 0 Superior tive 32 (0.03) Inferior (0.4) Inferior (2.0) (0.4) Inferior (2.6) (3.4) 1 Good Examples 33 (0.04) Inferior 2.9 Good 5.0 (0.7) Inferior (3.8) (2.9) 1 Good 34 (0.22) Inferior 3.3 Good 6.5 1.4 Good 0.3 0.5 (6) Inferior 35 (0.21) Interior 3.2 Good 6.0 >1.5 Superior 0.3 0.4 (7) Inferior 36 (0.21) Inferior 3.8 Good 8.5 >1.5 Superior 0.3 0.5 (7) Inferior 37 (0.23) Inferior 2.7 Good 5.0 >1.5 Superior 0.4 0.8 (8) Inferior 39 0.05 Good 1.4 Good 3.5 (0.9) Inferior (3.2) (2.5) 1 Good 39 0.06 Good 2.2 Good 4.0 (0.8) Inferior (3.4) (2.5) 1 Good 40 0.07 Good 3.3 Good 6.0 (0.9) Inferior (1.3) (2.6) 2 Good 43 0.05 Good 1.2 Good 3.5 (0.9) Inferior (1.2) (2.4) 1 Good 42 0.13 Good 2.7 Good 5.0 >1.5 Superior 0.3 0.5 (7) Inferior 43 0.17 Good 2.8 Good 5.0 >1.5 Superior 0.3 0.4 (6) Inferior 44 0.16 Good 2.8 Good 5.5 >1.5 Superior 0.4 0.8 (6) Inferior 45 0.08 Good 3.0 Good 5.5 (0.9) Inferior (1.3) (2.3) 5 Acceptable Value in ( ) means out of range of the present invention.

As shown in Table 5, in tests Nos. 1 to 18 in which all compositions satisfied the range of the present invention, time required until CPT reached 60° C. was not less than 1 hour, and superior delaying effect of deterioration of corrosion resistance was exhibited. Furthermore, crystal grain size of all of them was in the range of 3.0 to 7.0. In tests Nos. 19 to 30, all compositions satisfied the range of the present invention, but formula (2) was less than 1.2 or more than 5.0, crystal grain size of all of them was out of the range of 3.0 to 7.0, and the time required for CPT to reach 60° C. was slightly more than 1 hour.

On the other hand, in tests Nos. 31 to 33 in which all compositions satisfied the range of the present invention, but formula (1) was less than 0.05, the time required for CPT to reach 60° C. was less than 1 hour. In these cases, area ratio of α phase precipitated of all of them was more than 1%, and their grain size was more than 2 μm.

Furthermore, in test No. 31, formula (2) was more than 5.0, crystal grain size was 9.0, which was extremely fine, and time required until CPT reached 60° C. was only 0.3 hour.

In test No. 32, formula (2) was less than 1.2, crystal grain size was 2.0, which was extremely coarse, and time required until CPT reached 60° C. was only 0.4 hour.

Furthermore, in tests Nos. 34 to 37 in which formula (1) was more than 0.20, time required until CPT reached 60° C. was not less than 1 hour, and superior delaying effect of deterioration of corrosion resistance was exhibited; however, the number of cracks generated on a side surface of the hot rolled plate was not less than 6, and it was decided they were impossible to use for processing at high temperatures.

Furthermore, in tests Nos. 38 to 41 in which formula (1) and formula (2) were satisfied, but content of one of Sn, B, P and Si was less than the range of the present invention, effect of delaying precipitation of the σ phase thereby could not be obtained sufficiently, time required until CPT reached 60° C. was less than 1 hour. The σ phase area ratio at this time was more than 1% and grain size was more than 2 μm in all cases.

Furthermore, in tests Nos. 42 to 44 in which content of one of Sn, B, P was greater than the range of the present invention, time required until CPT reached 60° C. was more than 1 hour, superior effects of delaying deterioration of corrosion resistance were exhibited, number of cracks generated on a side surface of a hot rolled plate was not less than 6, and it was decided it was impossible to use them in processing at high temperature.

In test No. 45 in which the content of Si was greater than the range of the present invention, the time required until CPT reached 60° C. was less than 1 hour. The σ phase area ratio at this time was more than 1%, and the grain size was more than 2 μm.

INDUSTRIAL APPLICABILITY

According to the present invention, even if steel is exposed to a temperature in a range in which a σ phase precipitates, deterioration of corrosion resistance can be restrained. Therefore, it can desirably be used as a counterpart material of clad steel which is rolled and joined with thick carbon steel, or a highly corrosion-resistant material which is used in a process or the like in which a line furnace for brazing is used. 

1. A highly corrosion-resistant austenite stainless steel consisting of, in mass %: C: 0.005 to 0.030%, Si: 0.05 to 0.30%, Mn: 0.05 to 0.40%, P: 0.005 to 0.050%, S: 0.0001 to 0.0010%, Ni: 22.0 to 32.0%, Cr: 19.0 to 28.0%, Mo: 5.0 to 7.0%, N: 0.18 to 0.25%, Al: 0.005 to 0.100%, Cu: 0.05 to 0.50%, W: not more than 0.05%, Sn: 0.0005 to 0.0150%, Co: 0.030 to 0.300%, B: 0.0005 to 0.0050%, Fe as a remainder and inevitable impurities, wherein the stainless steel satisfies the following formula (1), an area ratio of a α phase is not more than 1%, and CPT based on ASTM G48 Method C as corrosion resistance property is not less than 60° C. 0.05≤10[% B]+2[% P]+6[% Sn]+0.03[% Si]≤0.20  (1).
 2. The highly corrosion-resistant austenite stainless steel according to claim 1, wherein the stainless steel further includes at least one of Nb: 0.005 to 0.250% and V: 0.005 to 0.250%, and satisfies the following formula (2), and crystal grain size of parent material based on JIS G0511 is in a range of 3.0 to 7.0 1.2≤100{2([% V]+[% Nb])+6[% B]}*([% N]+[% C]−0.1[% Mn])≤5.0  (2).
 3. A method for production of the highly corrosion-resistant austenite stainless steel according to claim 1, as a heat history after solution heat treatment, a temperature range of 700 to 1000° C. is maintained for 10 to 60 minutes by isothermal holding, cooling, or heating process.
 4. A method for production of the highly corrosion-resistant austenite stainless steel according to claim 2, as a heat history after solution heat treatment, a temperature range of 700 to 1000° C. is maintained for 10 to 60 minutes by isothermal holding, cooling, or heating process. 